1. Field of the Invention
The present invention relates to BJTs and, more particularly, to a method of forming a BIT with ESD self protection.
2. Description of the Related Art
A bipolar junction transistor (BJT) is a well-known element that is utilized in a variety of circuits. BJTs are commonly formed by sandwiching a region of a first conductivity type, known as a base, between two regions of a second conductivity type, known as an emitter and a collector.
FIG. 1 shows a cross-sectional view that illustrates a prior-art BIT 100. As shown in FIG. 1, BJT 100 includes a p− substrate 110, and a buried layer 112 that is formed in p− substrate 110. Buried layer 112 includes an inner n+ layer 112A and an outer diffused n layer 112B that extends out from inner n+ layer 112A.
Further, BJT 100 includes an n− epitaxial layer 114 that is formed on buried layer 112. BJT 100 is a high-voltage device which, when compared to a conventional low-voltage bipolar device, has a substantially thicker epitaxial layer. For example, n− epitaxial layer 114 can be approximately 15-17 um thick.
In addition, BJT 100 includes a p− base region 120 that is formed in n− epitaxial layer 114, an n+ emitter region 122 that is formed in p− base region 120, and a sinker down region 124 that is formed in n− epitaxial layer 114. Sinker down region 124 includes an inner n+ region 124A and an outer diffused n region 124B that surrounds inner n+ region 124A.
Sinker down region 124, along with n-type buried layer 112 and n− epitaxial layer 114, function as the collector. (N+ sinker down region 124A can alternately extend down to contact n+buried layer 112A, be combined with an n+ sinker up region that extends up from n+ buried layer 112A, or be implemented in any conventional manner.)
As further shown in FIG. 1, BJT 100 also includes a layer of isolation material 130 that is formed on the surface of n− epitaxial layer 114, and a metal base contact 132 that is formed through isolation layer 130 to make an electrical connection with p− base region 120. BJT 100 additionally includes a metal emitter contact 134 that is formed through isolation layer 130 to make an electrical connection with n+ emitter region 122, and a metal collector contact 136 that is formed through isolation layer 130 to make an electrical connection with sinker down region 124. Further, p− base region 120 is separated from collector contact 136 by a separation distance SD.
For normal operation, n+ emitter region 122 is commonly connected to ground, while n+ collector region 124 is connected to a positive voltage. Under these biasing conditions, BJT 100 is turned off when ground is placed on p− base region 120. In this case, the voltage on p− base region 120 is equal to the voltage on n+ emitter region 122, and less than the voltage on n+ collector region 124, thereby reverse biasing the base-collector junction.
On the other hand, when the voltage on p− base region 120 rises to approximately 0.7V, BJT 100 turns on. In this case, the voltage on p− base region 120 forward biases the base-emitter junction. When the base-emitter junction becomes forward biased, p− base region 120 begins injecting holes into emitter region 122, while n+ emitter region 122 begins injecting electrons into base region 120. The electrons injected into p− base region 120 diffuse through the lightly-doped base region 120, and are swept into n− epitaxial layer 114 by the electric field across the reverse-biased, base-collector junction.
Once swept into n− epitaxial layer 114, the electrons follow the lowest resistance path to n+ collector region 124. In this example, the lowest resistance path is illustrated by a current path P that moves vertically down, horizontally through n+ buried layer 112A, and vertically up to sinker down region 124. Normal operation continues as long as holes can continue to be supplied to p− base region 120 (for injection into n+ emitter region 122) via an external base current that flows into base region 120.
In addition to normal operation, BJT 100 can also be utilized to provide the pads of a semiconductor device with electrostatic discharge (ESD) protection from voltage spikes. For example, n+ collector region 124 can be connected to an I/O pad to protect the I/O pad from voltage spikes.
During an ESD event, the voltage on n+ sinker down region 124 rises quickly, which causes the voltage on n-type buried layer 112 and n− epitaxial layer 114 to rise with respect to the voltage on p− base region 120, thereby reverse biasing the pn junction between n− epitaxial layer 114 and p− base region 120.
When the rising voltage on n− epitaxial layer 114 (the collector) exceeds a breakdown voltage of the pn junction, avalanche multiplication causes large numbers of holes to be injected into p− base region 120, and large numbers of electrons to be injected into n− epitaxial layer 114. Ideally, the electrons injected into n− epitaxial layer 114 follow the same low-resistance path P to sinker down region 124 as described above.
On the other hand, the holes injected into p− base region 120 flow out of p− base region 120 into a circuit which causes the potential on p− base region 120 to rise and forward bias the base-emitter junction. For example, when a BJT is utilized as an ESD protection device, the base of the BIT can be connected to ground via a resistor. In this case, when the hole current from p− base region 120 flows to ground via the resistor, the resistor causes the voltage on p− base region 120 to rise and forward bias the base-emitter junction.
When the base-emitter junction becomes forward biased, p− base region 120 begins injecting holes into n+ emitter region 122, while n+ emitter 122 begins injecting electrons into p− base region 120. The electrons injected into p− base region 120 from n+ emitter region 122 diffuse to the base-collector junction, where the electrons are swept into n− epitaxial layer 114 by the electric field across the reverse-biased junction. The electrons from n+ emitter region 122 join the avalanche-generated electrons flowing to sinker down region 124, thereby significantly increasing the current sunk by BJT 100.
As noted above, the electrons injected into n− epitaxial layer 114 ideally follow the low-resistance current path P to sinker down region 124. However, due to the high electric field that is present during an ESD event, and the large number of electrons that are injected into the lightly-doped epitaxial layer 114, the electrons can flow laterally just below the surface of epitaxial layer 114 from p− base 120 to sinker down region 124.
As illustrated in FIG. 1, one problem with a significant lateral electron flow at the surface of n− epitaxial layer 114 is that the electron flow causes the lattice temperature to rise significantly, and can cause a localized hot spot 140 to develop next to the interface between sinker down region 124 and collector contact 136. Hot spot 140 can cause collector contact 136 to melt which, in turn, can lead to the electrical inoperability of BJT 100. Thus, there is a need for a BJT which can be used as an ESD protection device without being melted by an ESD event.